Liquid crystal displays (LCDs) are optical displays used in devices such as laptop computers, hand-held calculators, digital watches and televisions. Some LCDs include a light source that is located to the side of the display, with a light guide positioned to guide the light from the light source to the back of the LCD panel. Other LCDs, for example some LCD monitors and LCD televisions (LCD-TVs), are directly illuminated using a number of light sources positioned behind the LCD panel. This arrangement is increasingly common with larger displays, because the light power requirements, to achieve a certain level of display brightness, increase with the square of the display size, whereas the available real estate for locating light sources along the side of the display only increases linearly with display size. In addition, some LCD applications, such as LCD-TVs, require that the display be bright enough to be viewed from a greater distance than other applications, and the viewing angle requirements for LCD-TVs are generally different from those for LCD monitors and hand-held devices.
Some LCD monitors and most LCD-TVs are commonly illuminated from behind by a number of cold cathode fluorescent lamps (CCFLs). These light sources are linear and stretch across the full width of the display, with the result that the back of the display is illuminated by a series of bright stripes separated by darker regions. Such an illumination profile is not desirable, and so a diffuser plate is used to smooth the illumination profile at the back of the LCD device.
Some LCD monitors and most LCD-TVs commonly stack an arrangement of light management films adjacent to the diffuser plate on the opposite side from the lamps. These light management films generally comprise collimating diffuser films, prismatic light directing films, and reflective polarizer films. Handling of these individual light management films to manufacture LCD displays is very labor intensive as some films are supplied with protective cover sheets which must be first removed and then each light management film placed in the back light unit of the LCD individually. Also, inventory and tracking of each film individually can add to the total cost to manufacture the LCD display. Further, as these light management films are handled individually there is more risk of damage to the films during the assembly process.
Currently, LCD-TV diffuser plates typically employ a polymeric matrix of polymethyl methacrylate (PMMA) with a variety of dispersed phases that include glass, polystyrene beads, and CaCO3 particles. These plates are thick and heavy while display manufacturers are always trying to thin the form factor and reduce the weight of displays. The diffuser plates often deform or warp after exposure to the elevated humidity and high temperature caused by the lamps which causes viewing defects in the displays. In addition, the diffuser plates require customized extrusion compounding to distribute the diffusing particles uniformly throughout the polymer matrix, which further increases costs.
A previous disclosure, U.S. Pat. Application No. 2006/0082699 describes one approach to reducing the cost of diffusion plates by laminating separate layers of a self-supporting substrate and an optically diffuse film. Although this solution is novel the need to use adhesives to laminate these layers together results in reduced efficiency of the system by adding light absorption materials. Also the additional processing cost to laminate the layers together is self-defeating. Also, this previous disclosure does not solve the issue of the thick form factor and heavy weight of the diffuser plate. Nor does this solution address the issue of warping of the diffuser plate.
Another previous disclosure, US Pat. Application No. 2006/0171164 describes an optical film structure or element disposed between LC panel and an illumination source. This optical element uses film fixing parts attached to the optical films which are further attached to film tension controlling members which are connected to a frame. This disclosure describes the significant advantages of such an optical element in terms of preventing distortion of the optical films by maintaining tension on the films. One disadvantage of this approach, however, is that some of the optical films to be used in a light management arrangement or stack may have poor dimensional stability in terms of shrinkage, thermal expansion, or creep. Films with poor or low dimensional stability can be prone to buckle somewhat even when tensioned. Also, US Pat. Application No. 2006/0171164 does not teach how one could eliminate the thick plate diffuser by replacing it with a diffuser film that has equivalent optical performance.
It is desirable to replace the diffuser plate with an optical diffuser film, which would have a much thinner form factor as well as significantly lower weight. Such a diffuser film must have dimensional stability as well as high optical transmission while maintaining a high level of light uniformization. Further, it is desirable for such a diffuser film to have additional heat insulation value to reduce the heat gain from the light sources to the LC layer above the diffuser. Voiding is a well-known means to achieve both the optical requirements and the insulation requirements of the diffuser film.
The diffuser film may also provide the structural support for itself and optionally for the other optical films typically used in the light management arrangement. Because some optical films may shrink significantly or expand significantly under heat or possess poor long-term dimensional stability, in terms of creep propensity when held under tension, it may be desirable to sandwich the optical films between two transparent films, one of which could be the diffuser film. Such films should shrink very little under heat and have a low thermal expansion coefficient and comprise materials that have very good long term dimensional stability under tension (low propensity to creep), such as biaxially oriented PET. It is this structural support requirement without significant shrinkage, thermal expansion, or creep that is the essence of the present invention.
Therefore, it the object of the present invention to provide an optical element comprising a stack of at least three juxtaposed films with at least one inner optical film and the two outer transparent films exhibiting relatively high dimensional stability, wherein a controlled tensile force is applied in at least one direction to the two outer films but not the inner film.
In a preferred embodiment the optical element provides the optical smoothing function of previous plate diffusers and optical film arrangements or stacks at a very low cost. The optical element is unique in that it provides a high level of optical function and meets surface flatness requirements under specified thermal and humidity testing even at low thicknesses. Typically the optical element comprises a supporting frame around the perimeter of the at least three juxtaposed films that are supported by a controlled tensile force being applied to the two outer films. Other embodiments of the invention include other light management films also constrained by the supporting frame and tensioned films. These other optical films can be placed between and supported by the two tensioned outer films.